This disclosure relates generally to a conditioning and delivery system for cryogenic liquid. The present disclosure is particularly adapted for, but not limited to, a vehicle-mounted tank for efficiently conditioning and delivering liquefied natural gas (LNG) to an engine. However, people skilled in the technology will understand that the present disclosure can be employed to condition and deliver other cryogenic liquids to a number of applications.
For the purpose of this application, cryogenic liquids include liquefied gases that boil at temperatures at or below −150° F. under normal atmospheric pressure. LNG is one example of a cryogenic liquid because it boils at −258° F. under normal atmospheric pressure. Because of this, most cryogenic storage tanks are of a double wall construction. An inner pressure vessel is typically supported within an outer vessel. Radiation shielding is usually placed in the space between the inner and outer vessels, and the space is placed under a high vacuum to provide effective insulation against heat transfer.
The goal of a cryogenic liquid delivery system is usually to provide pressurized, gaseous material to an application, such as an engine, from a cold liquefied store of such material; however, a given volume of liquid produces many times the volume of gas when the liquid is vaporized. Because of this, systems that provide high pressure gas to an application from a relatively low pressure liquid have been developed. In common practice today, there are two methods for transferring pressurized cryogenic liquid from a cryogenic tank to an application such as an engine.
The first method for transferring cryogenic liquid from a storage tank to an application is a simple “pressure fed system”, which uses the pressure in the vapor space above the liquid in a storage tank to move cryogenic liquid into a vaporizer, where the liquid is heated to a pre-determined temperature suitable for use as a pressurized gas.
However, pressure in the vapor space above liquid in a storage tank must be sufficiently high to move liquid into the vaporizer. Because of this, a simple pressure fed system typically includes a system for “pressure building.” Pressure building systems typically rely on gravity, and use the liquid head pressure of the cryogenic liquid to move the liquid into an additional heat exchanger, where the liquid is vaporized, and then returned to the vapor space of the tank, thereby raising the pressure in the vapor space of the tank. However, pressure building systems have several problems.
One problem with pressure building systems is vapor pressure drop when demand for liquid is too great. Thus, if liquid is withdrawn from the storage tank at a high rate, the vapor pressure may be lowered to an unacceptable value due to an increase in ullage space. The circulation of cryogenic liquid through the pressure building heat exchanger is typically unable to compensate for this pressure drop, since the rate of pressure increase through the pressure build system is typically less than the rate of pressure loss during the withdrawal of liquid.
Another problem with pressure building systems is that there may only be a small liquid depth available in the tank to generate liquid head pressure to drive a pressure building circuit. Pressure drop in the heat exchanger and piping components is typically large enough that the liquid head pressure in the tank cannot overcome the resistance to flow, resulting in low or no flow through the pressure building circuit, and therefore resulting in no pressure increase in the tank.
Still another problem with pressure build systems is “pressure collapse.” Pressure collapse is caused by any agitation to the cryogenic tank, which causes condensation of vaporized liquid, resulting in a “collapse” or loss of pressure created during pressure building. While pressure collapse is not a problem in stationary cryogenic tanks, it is a major problem in mobile cryogenic tanks, such as those on vehicles.
Lastly, adding a pressure build system to a cryogenic liquid delivery system requires the entire system to contain two separate heat exchangers; the first is used in a pressure building system, and the second is the vaporizer used to supply pressurized gas to an application. Thus there are numerous fittings and connectors required to join these components, each of which is a potential failure point, compromising the reliability of the system.
One way of dealing with the many problems of pressure build systems, is to pump cryogenic liquid with an elevated saturated pressure into the tank. Saturated pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The saturated pressure of any substance increases non-linearly with temperature. Thus, cryogenic liquid with an elevated saturated pressure is created by elevating the saturated temperature of cryogenic liquid. This “warmed” cryogenic liquid, with an elevated saturated pressure is known as “conditioned liquid.” Conversely, “unconditioned liquid” has a relatively low saturated temperature and saturated pressure to conditioned liquid. If conditioned liquid is initially dispensed into the cryogenic tank, there is no need for a pressure build system because the pressure of the cryogenic liquid in the tank cannot drop below its already elevated saturated pressure. However, raising the saturated pressure and temperature of a cryogenic liquid also causes the liquid to expand and become less dense. Thus, conditioned liquid contains less energy per volume than unconditioned liquid. For example, a natural gas powered vehicle will be able to run substantially farther on a given volume of colder, unconditioned liquid than it will on the same volume of warmer, conditioned liquid.
The second method for transferring cryogenic liquid from a storage tank to an application is a “pump system” which uses an external pump to physically pressurize the cryogenic liquid and move it to an application. The pump elevates the pressure of the liquid and delivers it to a heat exchanger known as a vaporizer, where the liquid is heated to a pre-determined temperature suitable for use as pressurized gas. An accumulator commonly follows the vaporizer thus allowing for a ready supply of gas to be stored at or near the approximate conditions required for the application. Because a pump is able to physically and rapidly condition cryogenic liquid, pump systems have no problems with pressure drop, and are able to utilize unconditioned liquid; however, pump systems create a number of new problems. A pump system requires a minimum of three outside components, namely a physically removed pump, heat exchanger, and accumulator. Numerous fittings and connectors are required to join together such a delivery system, each of which is a potential failure point or leak path compromising the reliability of such a system.
One way of dealing with such space and reliability issues is to incorporate a pump into a cryogenic storage tank. A concern with introducing a pump directly into a storage tank is that it may create a heat leak, thereby reducing the holding time of the cryogenic liquid. “Heat leak” is a concern because as the liquid heats up it expands, which increases the pressure within the storage tank. Once the pressure in the storage tank becomes too high, a pressure relief valve will typically open, releasing a portion of the tank's contents into the atmosphere or to a recovery system. “Holding time” describes the time span that a cryogenic liquid can be held inside a storage tank before the pressure relief valve opens. Because high heat leak leads to shorter holding times, heat leak in a storage tank will result in venting off a substantial portion of gaseous cryogenic material if the tank is required to hold the liquid for any appreciable amount of time. For example, if a storage tank used to store LNG fuel for use in a vehicle, any natural gas that is vented off because of heat leak is fuel that was paid for by the operator but never used, increasing cost. It is therefore important for storage tanks to have relatively long holding times. Cryogenic storage tanks with low heat leak and relatively long holding times are said to have good “thermal performance.”
Historically, cryogenic liquid delivery systems either have the ability to effectively provide pressurized gas to an application at the expense of poor thermal performance, or good thermal performance with problems providing pressurized gas to an application. It is therefore desirable to provide a system that can effectively provide pressurized gas to an application, from unconditioned cryogenic liquid, to an application, and good thermal performance. Additionally it is desirable to provide a system with limited external components to ease installation on an application, and limit the number of potential failure points and leak paths.